In some implementations, a radio terminal is configured to directly communicate with other radio terminals. Such communication is called device-to-device (D2D) communication. The D2D communication includes at least one of direct communication and direct discovery. In some implementations, a plurality of radio terminals supporting D2D communication form a D2D communication group autonomously or under the control of a network, and perform communication in the formed D2D communication group.
A Proximity-based services (ProSe) direct discovery and ProSe direct communication specified in the 3GPP Release 12 are examples of the D2D communication (see, for example, Non-patent Literature 1). ProSe Direct Discovery is performed through a procedure in which a radio terminal capable of performing ProSe (i.e., ProSe-enabled UE) detects another ProSe-enabled UE by using only the capability of a radio communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology) possessed by these UEs. ProSe direct discovery may be performed by three or more ProSe-enabled UEs.
ProSe direct communication enables establishment of a communication path between two or more ProSe-enabled UEs existing in a direct communication range after the ProSe discovery procedure is performed. In other words, ProSe direct communication enables a ProSe-enabled UE to directly communicate with another ProSe-enabled UE, without communicating through a Public Land Mobile Network (PLMN) including a base station (eNodeB). ProSe direct communication may be performed by using a radio communication technology that is also used to access a base station (an eNodeB) (i.e., E-UTRA technology) or by using a wireless local area network (WLAN) radio technology (i.e., IEEE 802.11 radio technology).
In the 3GPP Release 12, a ProSe function communicates with a ProSe-enabled UE through a Public Land Mobile Network (PLMN) and assists ProSe direct discovery and ProSe direct communication. The ProSe function is a logical function that is used for PLMN-related operations required for ProSe. The functionality provided by the ProSe function includes, for example: (a) communication with third-party applications (a ProSe Application Server), (b) authentication of a UE for ProSe direct discovery and ProSe direct communication, (c) transmission of configuration information for ProSe direct discovery and ProSe direct communication (e.g., ProSe Application ID, ProSe UE ID, ProSe Layer-2 Group ID) to a UE, and (d) provision of network-level discovery (i.e., EPC-level ProSe discovery). The ProSe function may be implemented in one or more network nodes or entities.